The present invention relates to a method of blast furnace operation utilizing selective recycling of peripheral gas stream wherein the number of outlet ports for peripheral gas stream and the position of the gas outlet ports relative to the inlet ports for reducing gas are optimalized and the content of nitrogen in the recycling gas and the content of nitrogen in the reducing gas to be injected are controlled within the respective predetermined ranges, whereby a blast furnace operation is ensured which is highly efficient and stable. (According to the present invention, what is termed as the peripheral gas stream is defined as a gas stream that flows in an area between the inner wall surface of a blast furnace and a seven-tenths point on the radius extending from the cross-sectional center of the blast furnace to the furnace wall.)
Many different types of recycling processes are known in the art in which the gas taken out from the top of the furnace or the furnace shaft and reformed into a gas suitable for reducing purposes after the necessary treatment, is injected into the blast furnace at the lower level thereof. They include, for example, (a) Wiberg process, (b) Purofer process, (c) the process disclosed in the published Japanese Patent Application (Publication No. 27127/71), (d) Bureau of Mines experiment and (e) Nixon process. However, though the processes of (a) through (c) above utilize the recycling of the furnace gas, the reducing gas is injected into the blast furnace through the ordinary tuyeres and therefore the injection of the gases is effected at a single level. Therefore, these processes do not belong to the prior art techniques to which the improvements according to the present invention pertain, since there is no big difference in nitrogen content between the composition of the gas in the central area of the furnace and that of the gas in the peripheral area of the furnace. As compared with these processes, those of (d) and (e) above are more closely related to the techniques to which the subject matter of the present invention pertains in that the injection of the reformed reducing gas is effected through inlet ports other than the ordinary tuyeres and that two injection levels are utilized. However, in the Bureau of Mines experiment of (d), the waste gas mixed up irrespective of whether the peripheral gas stream or central gas stream, is removed and recycled for the purpose of regulating the temperature of the reducing gas to be injected. On the other hand, the Nixon process of (e) is designed so that the whole quantity of the waste gas is passed through a purifier and a portion of the purified gas is recycled. In these processes, therefore, the so-called peripheral gas stream flowing in the peripheral area of the furnace top or the furnace shaft is not selectively recycled.
In general, while nitrogen contained in the furnace gases serves as a carrier of heat in the process of heat exchange between the gases and the charge, it is inert in the reduction of ore. Therefore, it is desirable that the nitrogen content of the reducing gas should be minimized so far as the heat exchange between the gases and the charge does not have any detrimental effect on the operation of the furnace. Accordingly, it is essential for improved rate of operation and reduction in fuel ratio to minimize the introduction of nitrogen in the injected gas. In the above-mentioned recycling processes using two level injection, however, a gas mainly consisting of air and containing a greater quantity of nitrogen is injected through the ordinary tuyeres at the lower injection level, while a reducing gas having a low nitrogen content is injected through the upper level inlet ports. Consequently, these gases do not completely mix with each other with the result that in the furnace section above the upper level gas inlet ports, the high nitrogen content gas predominates the central area and the low nitrogen content gas predominates the peripheral area. A method of blast furnace operation in which such a low nitrogen content peripheral gas stream is selectively recycled and utilized as a reforming source to produce a reformed reducing gas that suits the desired blast furnace operation, has been proposed in the published Japenese Pat. Application (Publication No. 49671/71) of the Applicant.
The inventors have conducted various researches in the methods of blast furnace operation which utilize recycling of the abovedescribed peripheral gas stream and have discovered that in practising this recycling of peripheral gas stream, there is a certain optimum range with respect to the number of outlet ports for the peripheral gas stream as well as the angular relation between the peripheral gas outlet ports and the inlet ports for reducing gas. Further, it has been found that by controlling the nitrogen content of the peripheral gas to be taken out and that of the reducing gas to be injected within respective predetermined ranges, a satisfactory and stable blast furnace operation could be ensured.